In a color printer that is capable of printing color images, all colors are expressed by appropriately mixing a set of three primary colors, that is, cyan (C), magenta (M), and yellow (Y) (subtractive color mixture). In a computer, a digital camera, or another apparatus that supplies a color original to the printer, all colors are expressed by appropriately mixing another set of three primary colors, that is, red (R), green (G), and blue (B) (additive color mixture). Printing a color image accordingly requires conversion of the color expression using R, G, and B into the color expression using C, M, and Y. A general-purpose computer or a special data converter is used for such color conversion. The resolution of the color original supplied to the printer is not always coincident with the resolution of the printer. The general-purpose computer or the data converter is also used to regulate the resolution of the color original to be coincident with the resolution of the printer.
A diversity of methods based on the Neugebauer theory have been known as the techniques of converting the color expression by additive mixture of R, G, and B into the color expression by subtractive mixture of C, M, and Y Computation based on the Neugebauer theory at every time of color conversion undesirably lowers the speed of conversion. The technique adopted in the color conversion with the computer or the data converter carries out theoretical computation with regard to various values, summarizes the results of the theoretical computation in the form of a conversion table, and refers to the conversion table to enable the quick color conversion.
The color conversion with reference to the same conversion table may cause a subtle variation in color expressed on printing sheet, due to a variation in ground color of the printing sheet and a difference in compatibility of the printing sheet with inks. For example, an image printed on yellowish printing paper has a little yellowish tint. The resulting expressed color may also have a variation according to the degree of ink blot. In order to enable the accurate expression of colors in the original and thereby attain the high-quality printing, a number of conversion tables should be provided corresponding to different types of printing paper. An appropriate conversion table is then selected among the number of conversion tables for the color conversion process.
Each conversion table, however, requires a large storage capacity. Storing all the potentially available conversion tables in the data converter undesirably makes the data converter bulky and increases the manufacturing cost thereof. Storing only a limited number of conversion tables in the data converter, on the other hand, may prevent the color conversion from being performed according to an appropriate conversion table. This may result in inaccurate reproduction of colors in the original and thereby lower the picture quality of the resulting printed image.
The object of the present invention is thus to solve the above problems found in the prior art and to enable appropriate conversion of image data.